With the development of advanced valve train technologies (e.g., MultiAir®), it is now possible to control the amount of air used for combustion in each individual engine cylinder. A vehicle equipped with such technology (often referred to as variable valve actuation technology) manages the torque and power delivered by the engine by varying the lift profile of intake valves without direct use of a throttle. Instead, air intake is controlled using electro-hydraulic components that include a valve tappet, moved by a mechanical intake cam, connected to the intake valve through a hydraulic chamber that is controlled by a solenoid valve. The vehicle's engine control unit provides optimum intake valve opening schedules throughout the operation of the engine.
Standard engine oil is used as the valve operating fluid in the variable valve actuated engines described above. At high engine speeds, the oil is pressurized and typically free from air or air bubbles, which is desirable. However, when the oil pressure drops, air can effervesce from the oil (via e.g., the oil gallery), causing the oil to become aerated, which is not desirable.
One known situation where oil aeration can occur is when the engine speed is returned to idle. During a return to idle maneuver, oil pressure drops and air can effervesce from the oil. It has been discovered that valve lift can be lost due to a return to idle, especially in conditions of high oil aeration. FIG. 1 is a graph illustrating example test results for an engine that was running at 4,000 RPM (revolutions per minute) for a period of time before initiating a return to idle maneuver (i.e., air injection was shut off along with a command to reduce the speed). Oil aeration in this example was set to 19% and engine idle speed was the typical 700 RPM.
The x-axis of the FIG. 1 graph represents the number of crankshaft revolutions since the return to idle was initiated. Likewise, the y-axis of the FIG. 1 graph represents the amount of lift a particular intake valve experienced since the return to idle was initiated. As can be seen, significant valve lift was lost once the return to idle was initiated. In the illustrated example, there was over a 2.5 mm loss. This significant loss of valve lift can cause the engine to stall, which is undesirable and dangerous. Accordingly, there is a need and desire for a mechanism to control an engine during a return to idle maneuver that will not cause significant valve lift loss when the engine's oil is aerated.